The present invention relates to methods and devices for measuring and calibrating angular positioning of a machine member about a rotary axis. More particularly, the present invention relates to methods and devices for measuring and calibrating angular positioning of a machine member about a rotary axis, wherein a compensation value is empirically derived for each of one or more commanded rotary positions, and wherein the compensation value is based upon an actual rotary position accurately measured for a given commanded rotary position.
With reference to FIG. 1, a conventional machine tool 1, for example, a horizontal machining center, typically includes a spindle 2 mounted in a spindle carrier 3 for high-speed rotation about an axis parallel to a spindle axis “Z”. The spindle carrier 3 is typically translatable in directions ωX, ωY parallel to perpendicular axes “X”, “Y”, respectively, to which spindle axis “Z” is likewise perpendicular. The spindle 3 includes a free end adapted to securely grip a tool 4 for unitary rotation therewith. The machine tool 1 typically includes a worktable 5 moveable in a direction parallel to the spindle axis “Z” and rotatable in a direction about an axis “B” parallel to axis “Y”. The worktable 5 is adapted to securely receive a pallet 6 thereon for unitary movement towards and away from the spindle in direction ωZ, as well as for unitary rotation therewith in direction ωB. A workpiece 7 affixed to the pallet 6, then, may be positioned opposite the tool 4 by movement of the worktable 5 in direction ωZ and may thereby present its surfaces to the tool 4 by rotation of the worktable 5 in direction ωB.
Movement of the spindle carrier 3 in directions ωX, ωY, as well as movement of the worktable 5 in direction ωZ and rotation of the worktable 5 in direction ωB, is controlled, for example, by a conventional computer numerical control (CNC) program, in a manner such that the workpiece 7 may be machined by the tool 4. Because the tool 4 and the workpiece 7 are moveable relative to one another along four axes of motion, namely, in directions ωX, ωY, ωZ along linear axes “X”, “Y”, “Z”, respectively, and in direction ωB about rotary axis “B”, such a machine tool 1 is said to be a “four axis machine tool”.
However, it is oftentimes desirable for the machine tool 1 to be provided with an additional axis of motion, that is, a so-called “five axis machine”. For example, referring to FIG. 2, the spindle 2 may comprise a base member 2a and a swivel member 2b which are pivotable relative to one another in a direction ωA about a swivel axis “A”, the swivel axis “A” typically being horizontal, and therefore, parallel to axis “X” and perpendicular to axis “Z”. Tool 4, then, may approach the workpiece 7 at many angles oblique thereto, thereby increasing the complexity of the machining operations which may be performed by the machine tool 1.
As with nearly all machining operations, however, positioning of the tool 4 relative to the workpiece 7 must be performed with a high degree of accuracy and repeatability. Presently, this is accomplished, in part, through the use of conventional CNC programs, which, along with conventional servomechanisms, linear motors and ballscrew-type drives, provide accurate and repeatable machine member motion. For example, a conventional CNC program may command a servomotor (not shown) coupled to the swivel member 2b to rotate the swivel member 2b in the direction ωA through an angular displacement of 30°. The CNC-controlled servomotor provides accurate and repeatable motion, relative to the initial and final positions of the swivel member 2b, typically accurate to within 3-10 arc-seconds.
According to one known method for calibrating the movement of a machine member about a rotary axis, a conventional bubble level is suspended from the machine member for rotation therewith about an axis parallel to the axis about which calibration is desired. A precision angular measuring device, such as an optical encoder, is connected to the level such that rotation of the level relative to the machine member can be measured. With the machine member in a first position, the level is manually positioned in an “on-level” orientation and the angular measuring device is “zeroed”. The machine member is then moved to a second position and the level is thereafter manually re-positioned to the “on-level” orientation, if necessary. The rotary displacement of the level is measured and compared to the desired rotation of the machine member to obtain an error compensation value. However, because this method requires manual positioning of the level, any error compensation values obtained thereby may include unavoidably inaccuracies. It is therefor desirable to provide a method for calibrating movement of a machine member about a rotary axis, wherein rotation of a calibration device used therefor is controlled.
It is desirable to provide a device and method for calibrating the rotary movement and positioning of a rotatable machine member.
It is furthermore desirable to provide a device and a method for acquiring rotary position data of a rotatable machine member and for empirically deriving compensation values to correct rotary positioning errors.
It is even furthermore desirable to provide a device and method for automatedly acquiring rotary position data of a rotatable machine member for each position of a plurality of positions defining a range of motion of the machine member.
It is also desirable to provide a device and method for acquiring rotary position data of a rotatable machine member oriented for rotation about a horizontal, vertical or oblique axis.
It is furthermore desirable to provide a device and method for generating an NC program to directly control movement and positioning of a machine member.